Electrostatographic single-pass multiple station printers are known in the art, such as from U.S. Pat. No. 4,977,411.
U.S. Pat. No. 5,499,093 describes an electrostatographic single-pass multiple station printer, which comprises a number of consecutive single colour printing stations. Each single colour printing station is adapted to print a specific single colour image to a printing medium passing though the printer. All single colour images printed one over the other provide together the composite image being a multicolour image. In order to generate a multicolour image without colours being in offset one versus the other, it is understood that a perfect timing or synchronisation of the printing actions of the consecutive printing stations is necessary.
For obtaining such synchronisation, U.S. Pat. No. 5,499,093 discloses that an encoding means is used to generate a set of pulses, which pulses are indicative of the web displacement that has occurred. The encoding means is driven by one of the rotatable endless surface means onto which a toner image can be formed at one particular printing station. The transformation of the set of pulses into synchronised commands for the consecutive printing stations is performed by a register control means.
According to U.S. Pat. No. 5,499,093 the encoder means is preferably coupled to a printing station which is not located as the first, nor as the last printing station in the sequence, but to an intermediate printing station, preferably the central printing station in the sequence of consecutive printing stations. Such a choice of an intermediate printing station is preferred because in that case the web path between the drum carrying the encoder and the drum most remote therefrom is minimised thereby reducing any inaccuracies which may arise from unexpected stretching of the web and of variations induced by the eccentricity of the drums or the rollers defining the wrapping angle ω, caused by e.g. machine vibrations. The high-frequency vibrations may be compensated or filtered with large accuracy by a filtering means being part of the register control means.
The register control means may possibly comprise manual adjustment means to compensate deviations of web displacement, measured after printing and analysing prints of calibration images.